Radio receivers that recover frequency modulated (FM) voice signals typically include a squelch circuit to mute the recovered voice signal when it is too weak or non-existent, and unmute when the recovered voice signal is likely to be intelligible. This is also described as determining whether there is channel activity. Such FM receivers typically recover an audio signal having a bandwidth greater than 10 kHz when they are activated. The recovered audio includes the recovered voice signal having a bandwidth of approximately 3 kHz. Typical squelch circuits employ a measurement of the energy of a portion of the spectrum of the recovered audio ("out of band audio") that extends from approximately 10 kHz to 30 kHz as a factor in determining whether to mute the recovered voice signal. This is based on the well known phenomenon that the energy in the 10 kHz to 30 kHz band of the receiver decreases in response to a stronger radio signal.
Many conventional voice radio receivers that are not designed for channel scanning employ continuous detection of the energy of the out of band audio, often combined with hysteresis techniques, to avoid muting a recovered audio signal during brief periods of weak signal caused by fading. The response time of such circuits are typically 100 milliseconds (msec) or more, and they work quite well.
However, when a voice radio receiver is designed for channel scanning of a plurality of voice channels, and a priority function is provided, a squelch circuit having a 100 millisecond response time is generally too long, since audio output will be interrupted and will be annoying to the listener. In a priority scanning function, the radio selects a secondary channel on which activity has been detected when no activity has been found on a priority channel, but repetitively checks the priority channel for activity. The checking causes an interruption of the recovered audio of the secondary (non-priority) channel while the recovered audio from the priority channel is being tested to determine if a valid signal is present on the priority channel. The interruption is continued after the channel is switched back to the secondary channel while the recovered audio from the secondary channel is checked to determined if a sufficiently good signal is still on the secondary channel. Thus it is particularly important in this type of scanning to have fast activity detect times. Even when a scanning receiver does not provide a priority scan function, a slow squelch response requires a slow scanning rate, which is less desirable to many customers.
One prior art technique for speeding up squelch detection is described in U.S. Pat. No. 5,128,826, issued to Masaki on Jun. 21, 1995, which is entitled "High-Speed Scanning Radio Receiver." In the technique described, channel activity is determined based on four types of signals. Two of the four types are generated from the out of band recovered audio. One of the two types is a "noise level" signal having two states (the "SC" signal), based on a detection of the energy in the out of band audio. A squelch state of the SC signal indicates that the channel has a sufficiently strong voice signal. When the SC signal is not in the squelch state, and a new channel having a sufficiently strong voice signal is selected, the SC signal eventually reaches a predetermined voltage threshold and the SC signal switches to the squelch state. However, the SC signal is "not so fast"--it can take up to 100 msec to change to the squelch state. The other signal of the two types, the "SP" signal, is also generated. It is in a first state (the active state) when the SC signal is changing towards the squelch state at a rate exceeding a predetermined rate. When a new channel is selected, if the SP signal is active within a predetermined time determined by a first timer, a second timer having a longer duration (e.g., 100 milliseconds) than the first timer is started. If the SC signal does not indicate voice activity by the expiration of the second timer, the next channel is sought (in a version of the technique described in cols. 2 and 3 of Masaki). In another version, when the described conditions occur, a state of the SP signal during the scan of the previous channel is used to determine whether the present channel has activity or a next channel should be scanned. While this technique may provide a fast determination of channel activity in many instances, it requires not only an out of band audio detector to generate the SC signal, but also a second circuit to generate the SP signal. It also sometimes waits 100 ms to make a decision, and can either miss detection of channels with activity (as asserted at the end of col. 1 and beginning of col. 2) or in the alternative (in the second version) waits 100 ms to make a determination.
Another prior art technique for speeding up squelch detection is described in U.S. Pat. No. 4,731,868 issued to Drier on Mar. 15, 1988, which is entitled "Circuitry in a Scanning Receiver for Speeding Up the Generation of a Reception or Non-Reception Criterion." In this technique, an out of band noise criterion is applied to two capacitors when the channel is switched. The capacitors are held, respectively, at a high and low voltage reference levels until the channel has switched, at which time the outputs of the capacitors change according to the amount of energy in the out of band signal, ultimately reaching the steady state value of the noise criterion (when a steady state value exists). The outputs of the capacitors are logically combined and produce two signals, only one of which can be active. One signal is an indication of valid voice activity and the other signal is an indication of noise. The time from the beginning of the measuring period until an active state of one of the two signals is reached is compared with a reference time and channel activity is determined by whether the active state is reached before that time. Although this technique may also provide a fast determination of channel activity in many instances, it is complex in that it requires two capacitor circuits when both channel activity and inactivity need to be quickly determined, and requires the measurement of non-predetermined times.
Thus, what is needed is a simpler, less costly, and reliable technique for determining the presence and absence of valid channel activity in a channel scanning selective call radio.